`set` triggers a strange warning when adding a `NA` to a factor column by reference

[DavidArenburg]

library(data.table) ## v1.9.5+
data = data.frame(cats = rep(c('', ' ',  'meow'), 5))
set(data, i = grep("^$|^ $", data[[1]]), j = 1L, value = NA_integer_)
# Warning message:
#   In set(data, i = grep("^$|^ $", data[[j]]), j = 1L, value = NA_integer_) :
#   RHS contains -2147483648 which is outside the levels range ([1,3]) of column 1, NAs generated

While base R handles it correctly

data = data.frame(cats = rep(c('', ' ',  'meow'), 5))
indx <- grep("^$|^ $", data$cats)
data$cats[indx] <- NA_integer_

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